Electrolytes considered for this invention are applied in layers and transformed to provide additional functions such as separating electrodes, holding position, preserving ionic conductivity, or bonding other layers of electrochemical cells. The electrolyte and electrode layers can be laid down by an in-line press in repeating patterns to manufacture a succession of thin, flexible, low cost, and low power electrochemical cells.
Printed electrochemical cells and batteries (multiple cells) are still relatively rare despite a number of published inventions relating to their manufacture, which involves printing at least some of their active layers and laminating others in sheet or web form. Some of the problems still affecting the success of printed electrochemical cells involve difficulties with printing effective electrolyte layers interconnecting layers of electrodes.
One early attempt at printing electrochemical cells is disclosed in U.S. Pat. No. 2,688,649 to Bjorksten. Electrode/electrolyte combinations are formulated as inks and laid down in repeating patterns by transfer printing, which includes letterpress or offset printing. Magnetic powders together with an electrolyte solution are suspended in a printing vehicle such as a drying oil or a resinous material. After printing, the ink is passed through an oriented magnetic field and dried. Another ink containing a different magnetic powder is printed over the first layer, magnetically oriented, and dried to complete a printed xe2x80x9cdry cellxe2x80x9d. Although printed, the electrolyte is printed together with the electrode powders, which limits cell configurations (e.g., side-by-side electrodes) and the ability of the electrolyte to function as a separator between the electrodes.
U.S. Pat. No. 3,230,115 to Tamminen discloses printed electrochemical cells in which a metallic zinc electrode and a carbonaceous electrode are laid down side-by-side in repeating patterns and covered by a porous material wetted with electrolyte in the form of a viscous adhesive gel. The electrolyte is a calcium chloride solution gelled by wheat flour. The suggestions for printing include applying the electrode layers by coating and impregnating a porous paper with the electrolyte before applying the paper to the electrodes. Two more recent examples of printed cells with porous separators impregnated with electrolyte are disclosed in U.S. Pat. No. 5,055,968 to Nishi et al. and U.S. Pat. No. 5,652,043 to Nitzan. Although absorbed by a separator, such liquid electrolytes are difficult to confine and are subject to evaporation.
A solid electrolyte layer separates electrode layers of a printed cell disclosed in U.S. Pat. No. 5,350,645 to Lake et al. Accordingly, the electrolyte must be laminated rather than printed and is limited to unusual and expensive materials that are solid but contain moveable ions. For example, solid lithium iodide is suggested as an electrolyte between a lead iodide cathode and a lithium anode.
Another example of a printed lithium cell is disclosed in U.S. Pat. No. 5,035,965 to Sangyoji et al. The proposed electrolyte is an ion-conductive polymer obtained by mixing polyethylene oxide with lithium salt. Screen printing is used to apply the polymer electrolyte to a metal foil electrode, and a so-called UV-calcinating oven dries the electrolyte into a solid form. Ordinary electrolytes are generally not ionically conductive in a solid form; and polymer based electrolytes, such as those disclosed in the Sangyoji et al. patent, are generally not useful for formulating printing inks of more rapid transfer printing operations, such as flexographic printing.
We propose the manufacture of electrochemical cells with an electrolyte that can be laid down as a liquid printable ink and subsequently transformed to perform additional functions such as separating electrodes, holding position, preserving ionic conductivity, or bonding cell layers together. For example, electrolyte formulations can be made that are particularly suitable for transfer or injection printing but can also be cured into an adhesive state.
One method of forming a succession of such electrochemical cells along an in-line press includes formulating an electrolyte composition containing both an electrolyte and a monomer. The electrolyte composition is printed in a succession of patterns on an advancing web and is subsequently transformed by converting the monomer into a polymer that forms a matrix within which the electrolyte is embedded. The successions of electrolyte and electrode patterns are arranged to form a succession of electrochemical cells along the web.
The electrolyte composition containing the monomer preferably has low viscosity and low adhesive characteristics consistent with conventional liquid printing ink and is adaptable to ink printing techniques such as transfer or injection printing. The transformation step increases both the viscosity and the adhesive characteristics of the printed electrolyte composition for performing a bonding function between other layers supported on the web. The resulting electrolytic adhesive holds position within the cell and is less susceptible to drying out.
Another method emphasizing the printing of electrochemical cells with electrolyte patterns having high-adhesive properties starts with an electrolyte composition that is formulated for having low adhesive properties. The electrolyte composition having low adhesive properties is printed in a repeating pattern along an advancing web. The repeating patterns are chemically transformed to exhibit high adhesive properties, which is useful for such purposes as bonding other cell layers together or separating overlapping electrode layers. The chemical transformation can involve polymerizing or crosslinking the electrolyte composition resulting, for example, in a patterned electrolyte that is also a pressure-sensitive adhesive.
Transfer printing can be used for printing electrochemical cells along an in-line press by separately formulating at least one electrode composition and an electrolyte composition in transfer inks. The electrode composition and the electrolyte composition are printed by successive printing stations of the in-line press in repeating patterns on at least one of two web layers. A curing station chemically transforms the electrolyte composition into an electrolytic pressure-sensitive adhesive that bonds the two web layers together and that completes at least a portion of an ionically conductive pathway between two electrodes of a progression of transfer-printed electrochemical cells.
Injection printing can also be used for printing similar electrochemical cells along an in-line press by formulating the electrolyte composition to permit pooling of the electrolyte in pre-formed reservoirs. A succession of the reservoirs is formed along an advancing web, and a periodic injection of a metered volume of the electrolyte fills the reservoirs. The electrolyte, which is injected in a flowable form, assumes the shape of the reservoirs by force of gravity. A subsequent curing step chemically transforms the electrolyte into a more permanent form, such as a pressure-sensitive adhesive.
The in-line manufacture of electrochemical cells in accordance with our invention can also include the laying down of more than one electrolyte layer. At least one web supporting anode and cathode layers in successions of patterns is advanced through an in-line press. A first layer of electrolyte is laid down in a succession of patterns on the anode layer, and a second layer of electrolyte is laid down in a succession of patterns on the cathode layer. The two electrolyte layers are cured while separately in contact with the anode and cathode layers. A laminating operation joins the two cured electrolyte layers together to complete ionically conductive pathways between the anode and cathode layers.
Curing individual electrolyte layers in contact with one or both electrode layers improves ionic conductivity between the electrolyte and electrode layers by eliminating surface formations that can block ion transfers. Individual layers of electrolyte can be later joined together without the same adverse consequences because of their natural affinity for each other. In addition, the enhanced flow characteristics of the applied electrolyte allow the electrolyte to conform to surface irregularities, particularly those of printed electrodes having rough or granular surfaces. The more intimate molecular contact between the electrolyte and electrode layers improves both bonding strength and ionic conductivity through the interface.
One version of an electrochemical cell arranged in accordance with our invention includes two electrode layers and an electrolyte composition laid out on at least one substrate. The electrolyte composition is chemically transformed by polymerization into a matrix structure containing an embedded electrolyte with disassociatable ions moveable between the electrode layers.
The electrolyte composition is preferably polymerized in contact with the one electrode layer forming an interface that promotes movement of ions between the one electrode layer and the electrolyte composition. The electrolyte composition can also be laid down in two layers that are separately cured in contact with the two electrode layers and that are later joined to complete an ionically conductive pathway between the electrode layers.